Patients with idiopathic membranous nephropathy are being studied to evaluate the efficacy and toxicities of the addition of intermittent cyclophosphamide to alternate day oral corticosteroid therapy. Efficacy will be judged by determinations of effective renal plasma flow, glomerular filtration rate and glomerular capillary wall permselectivity performed with dextran and urine protein (albumin and immunoglobulin) clearance techniques. Kidney biopsy morphology (including morphometric analysis) will be examined at the beginning and at the end of treatment as part of detailed studies of structure-function relationships and the efficacy of various therapeutic modalities. Patients with membranous nephropathy and 2 or more grams per day of proteinuria will be treated with alternate day prednisone and will be randomized to receive: a) no additional therapy (control group), or b) intravenous pulse cyclophosphamide up to 1.0 gram per square deter body surface area every other month for 6 total doses. Analysis will include comparison of the number of favorable outcomes of glomerular function and pathology, as well as drug-related toxicities observed in each treatment group at the end of the 12 months of study. To date, the rates of renal outcomes are not significantly different in the groups receiving prednisone alone or prednisone plus intermittent cyclophosphamide.